High temperature resistant coating

ABSTRACT

A composition for coating steel components, such as the compressor parts of gas turbine engines consisting essentially of from two parts to half a part by weight of potassium silicate; from one part to two parts by weight of finely divided aluminum powder, from one quarter to three quarters of a part by weight of a polyhydroxy alcohol and sufficient water to produce a viscosity of up to and including forty seconds when measured on a British Standard number 4 flow cup at 20* C.

United States Patent 1 Ballard Dec. 4, 1973 [54] HIGH TEMPERATURE RESISTANT 3,656,975 4/1972 Phelps et a1. 106/84 ING 1,771,605 7/1930 Bailey 2,926,098 2/1960 llenda et a]. 106/74 [75] inventor: Norman Edmund Ballard, Derby,

England [73] Assignee: Rolls-Royce (1971) Limited, Derby, m ry mine -James E- POer England Attorney-John W. Malley et a1.

[22] Filed: Feb. 10, 1972 [21} Appl. No.: 225,062

Related u.s. Application mm 1 1 ABSTRACT [63] Continuation-impart of Ser. No. 136,537, April 22,

211 abandoned. a A composition for coating steel components, such as 5 the compressor parts of gas turbine engines consisting [30] Foreign Application Priority Data essentially of from two parts to half a part by weight of Apr. 24, 1970 Great Britain 19,761/70 PQE Silicate from One P to P y Wiigli" oi finely divided V v der, from one 52 US. Cl. 106/84 quarter to three quarters fi' y weight w- 51 Int. Cl C09d 1/02 11% and twat to produce a [58] Field of Search 106/74, 84 cosity of p to and induding forty Seconds when sured on a British Standard number 4 flow cup at 20 [56] References Cited UNITED STATES PATENTS 3,423,229 1/1969 Kompanek et a1 106/84 3 Claims, N0 Drawings 1 HIGH TEMPERATURE RESISTANT COATING This application is a continuation-in-part of applicants application Serial No. 136,537, now abandoned.

This invention relates to high temperature resistant coatings.

It is sometimes necessary to protect metal components against corrosion by atmospheric constituents, and a paint coating is frequently chosen to provide such protection. This problem becomes more difficult when the components concerned operate at elevated temperatures, and may be particularly difficult to overcome when the atmosphere contains a relatively high proportion of salt. A particular instance where this combination of circumstances arises is in compressor parts of gas turbine engines which spend a high proportion of their operating time in marine conditions.

The present invention relates to a coating which has been found to be highly efficacious under such circumstances.

According to the present invention, a composition for coating steel components consists essentially of from two parts to half a part by weight of potassium silicate; from one part to two parts by weight of finely divided granular aluminium powder; from one quarter to three quarters of a part by weight of polyhydroxy alcohol and sufficient water to produce a viscosity of up to .and including 40 seconds when measured on a British Standard number 4 flow cup at 20C.

Where the coating is intended to be sprayed, the preferred composition is one part by weight of a 50 percent aqueous solution of potassium silicate having a mean weight ratio SiO,:K O of between 1.43 and 2.05; one and one quartefpar tsbyweight of final {divided granular aluminium powder; half a part by weight of glycerol and sufficient water to produce a viscosity of LII from 15 to 25 seconds when measured on a British Standard number 4 flow cup at 20C.

However, when the coating composition is to be electrophoretically applied we prefer to use one part by weightof a 50 percent aqueous solution of potassium silicate having a mean weight ratio SiO :K O of between ].43 and 2.05; two parts by weight of finely divided granular aluminium powder; halfa part by weight of glycerol and one and a half parts by weight of water.

The preferred method of application of the coating composition comprises air drying the paint coating and subsequently heat treating it from 150C to 190C for 2 hours and at 560C for 2 hours.

In an example of a method according to the present invention the coating composition was made up by mixing one part by weight of a 50 percent aqueous solution of potassium silicate having a mean weight ratio SiO K,O of 1.05 with one and one quarter parts by weight of finely divided granular aluminium powder having a particle size of between 5 and 10 microns and being wax free, half a part by weight of glycerol and sufficient water to produce a viscosity of 16 seconds when measured with a British Standard number 4 flow cup at 20C.

The potassium silicate was that supplied by Crossfield Chemicals Limited of Warrington England under the designation Grade 66." We believe however that other grades of potassium silicate supplied by Crossfields, namely grades 70, 82, 84 and 120 having mean weight ratios sio,;K,0 of 2.05, 2.01, 1.89 and 1.43 respectively, would be equally suitable for use in the coating composition.

The glycerol is added to the coating composition in order to adjust the solvent balance thereof so as to achieve optimum spraying properties and to provide a more thorough cure. Although glycerol is used in the present example, it will be appreciated that other polyhydroxy alcohols such as glycol would be as equally effective.

The coating composition was sprayed on to test panels of 12 percent chromium steel. The wet coating was then air dried for 15 minutes to give a coating of 1% to 2 thousandths of an inch in thickness. The test panels were then heat treated between C and C for two hours and subsequently further heat treated for two hours at 560C. The test panels were used to determine the coatings resistance to lubricating oils, hydraulic fluids, oil fuels and various coolants and the coating was found to be substantially unaffected by contact with these liquids. The panels were next tested for their resistance to dry heat and it was found that the coating resisted temperatures up to som e 6 50C. Similar tests involving heating at 450C followed by exposure to salt spray showed that the panels had satisfactory resistance to marine atmospheres, and tests of a similar nature using panels with substantially damaged coatings indicated that a sacrificial electrolytic protection was given by the coating to uncoated areas. Final erosion tests were carried out using a carefully controlled sandblasting technique, and test panels using the present coating composition were found to be at least as good as any commercially available coating.

In a second series of examples a different coating compositions mixture was used; in this case one part by weight of the 66 grade" silicate solution was mixed with two parts by weight of finely divided aluminium powder having a particle size of between 5 and 10 microns, one-halfa part by weight of glycerol and one and a half parts by weight of water. In this case similar panels were coated by electrophoresis; that is they were immersed in the liquid and an electric current between a V4 and 1 amp per square inch passed between the panels and the liquid for some 178 to 1% minutes. This coating composition and deposition treatment gave a coating of between 4/5 and 2 thousandths of an inch which was otherwise very similar to that produced in the first set of examples, and which was heat treated under similar conditions and gave very similar test results. We believe that the difference in coating composition constituents is necessary because of the differing mobilities of the constituents when applied electrophoretically.

It will be appreciated that various modifications could be made to the constituents within the ranges set out above, and that although the examples used a particular material for the test panels, it will be possible to use a variety of steels.

I claim:

1. A high temperature resistant composition for coatin steel com onents consisting essentially of from two parts to half a part by weight of potassium silicate; from.

one part to two parts by weight 0 mely ivided granular aluminium powder; from one quarter to three quarters of a part by weight of a polyhydroxy alcohol and sufficient water to produce a viscosity of up to and including forty seconds when measured on a British Standard number 4 flow cup at 20C.

3. A composition as claimed in claim 1 and which is adapted to be applied electrophoretically consisting essentially of one part by weight of a 50 percent aqueous solution of potassium silicate, which potassium silicate has a mean weight ratio SiO,:K O of between 1.43 and 1.05; two parts by weight of finely divided grannular aluminium powder; half a part by weight of glycerol and one and a half parts BY weight of water.

a t It t 

2. A composition as claimed in claim 1 and which is adapted to be applied as a spray consisting essentially of one part by weight of a 50 percent aqueous solution of potassium silicate, which potassium silicate has a mean weight ratio SiO2:K2O of between 1.43 and 2.05; one and one quarter parts by weight of finely divided granular aluminium powder; half a part by weight of glycerol and sufficient water to produce a viscosity of from 15 to 25 seconds when measured on a British Standard number 4 flow cup at 20*C.
 3. A composition as claimed in claim 1 and which is adapted to be applied electrophoretically consisting essentially of one part by weight of a 50 percent aqueous solution of potassium silicate, which potassium silicate has a mean weight ratio SiO2:K2O of between 1.43 and 1.05; two parts by weight of finely divided grannular aluminium powder; half a part by weight of glycerol and one and a half parts BY weight of water. 